Brain aging processes are enormously complex phenomena that affect multiple systems, cell types and pathways, and result in cognitive decline and increased risk of Alzheimer's disease (AD). Landfield P W et al., J Neurobiol 23: 1247-1260 (1992). Although several biological mechanisms have been putatively linked to brain aging or Alzheimer's disease, including inflammation, oxidative stress, Ca2+ dyshomeostasis (Landfield, P W & Pitler T A, Science 226: 1089-1092 (1984); Landfield P W et al., J Neurobiol 23: 1247-1260 (1992)), mitochondrial dysfunction and chronic exposure to adrenal stress hormones (Landfield P W et al., Science 214: 581-584 (1981); Porter N M & Landfield P W, Nature Neurosci 1: 3-4 (1998)), the specific mechanisms and pathways, if any, through which they are linked to impaired brain function are not understood.
It is widely thought that gene expression changes contribute to many aspects of declining function with aging. Finch C E, Longevity, Senescence and the Genome, 37-42 (Univ. Chicago Press, Chicago, 1990). It is also thought that gene expression changes are important for processing and storage of memory. However, not all genes that change expression in the brain with aging are thought to be important for cognition.
Gene-expression changes that specifically contribute to age-related memory decline should selectively change with brain aging and should be correlated specifically with measures of age-associated cognitive decline; that is, a subset of the full set of aging-dependent genes should also correlate with age-related cognitive decline. See, Lockhart D J & Barlow C, Nat Rev Neurosci 2: 63-68 (2001) and Mirnics K, Nat Rev Neurosci 2: 444-447 (2001).
If a subset of age-dependent genes also shows expression patterns directly correlated with age-related memory decline, then such a subset of “aging and cognition-related genes” (ACGs) would be extremely helpful as biological indexes (“biomarkers”) for assessing or diagnosing the degree of age-related cognitive impairment in individual subjects. In turn, the ability to measure aging-related cognitive impairment quantitatively is essential for discovering new therapeutic targets, and developing new strategies and pharmaceutical compounds for counteracting normal age-related cognitive decline and/or age-related neurodegenerative diseases, including Alzheimer's disease (AD) or Parkinson's disease (PD).
Identifying ACGs in any mammalian species therefore, might have great therapeutic usefulness. Moreover, because of the well-established homologies of most genes across mammalian species and because of the clear similarities in patterns of brain aging and cognitive decline across species, identification in any mammal would have human health implications. Furthermore, because the primary risk factor for Alzheimer's disease and Parkinson's disease is aging itself, therapeutic approaches developed for aging-related cognitive impairment should also help ameliorate cognitive decline from age-related neurodegenerative disease. Thus, there is a clear need for identifying ACGs but, to date, such genes have not been discovered for any mammal.
Gene microarray technology provides a powerful approach for unraveling the complex processes of aging. To date, however, its impact has been limited by statistical problems, small sample sizes, and difficulty in assessing functional relevance. Moreover, studies that have examined gene expression during brain aging using microarrays have not used sample sizes large enough to provide adequate statistical power for formal statistical testing. Lee C K et al., Nature Genetics 25: 294-297 (2000); Jiang C H et al., Proc Natl Acad Sci USA 98: 1930-1934 (2001) Therefore, even the genes they have reported to change with aging have not been validated by accepted statistical criteria.
The extremely large data sets generated by microarrays pose formidable bioinformatics and resource problems that have to date limited the impact of this powerful technology. Because of these difficulties, most microarray studies have relied on simple fold change comparisons in small samples. However, neither fold change analyses nor the small sample protocols widely used allow the direct estimates of variance necessary for defining type I error (false positives). In addition, fold change criteria, by definition, select for large changes. Therefore, they exhibit low detection sensitivity (high false negatives, or type II error), and are unable to identify the modest changes that often characterize functionally important (and, therefore, tightly regulated) genes. The inability to assign type I error is a particularly critical problem for microarray studies because the thousands of comparisons of gene expression in such analyses greatly increase the expected false positives. For example, even if group sizes were sufficient for formal statistical analyses, and 5000 gene transcripts were each tested by t-test for differences between two conditions at p≦0.05, the false positive rate is equal to the p-value and, consequently, 5% of the 5000 tested transcripts (250) would be expected to be found significant by chance alone.
Although microarray studies have some important offsetting advantages that improve statistical confidence (e.g., co-regulation of genes within a functional group), there is increasing recognition that microarray experiments should generally meet the same statistical standards as other biological experiments or, at least, should systematically estimate the degree of statistical uncertainty. Several strategies to improve statistical confidence have been developed for small-sample microarray studies, but these generally rely on indirect estimates of variance and/or greatly sacrifice sensitivity (i.e., stringent p-values).
Another highly important problem of microarray studies is that of determining which of the hundreds of expression changes that may be observed are likely to be functionally relevant. Correlation analysis is one quantitative approach to linking gene expression with function, although it also requires relatively large sets of independent samples. Expression-function correlations fulfill a key prediction of a causal relationship (i.e., that causally related variables should co-vary) and therefore, can serve as a valuable tool for the identification of candidate functionally relevant genes. Nonetheless, there have been few correlation studies attempting to link cognitive dysfunction with univariate gene expression patterns across individual subjects, much less using the massive amounts of data generated in microarray analyses.